There are active implantable medical devices which are known to adapt their actions, for example, the stimulation frequency, to a measured or calculated value of a parameter that is representative of the metabolic needs of the person in which the device is implanted. One sensor which is generally used to measure such a parameter is the so-called minute volume or minute ventilation sensor (often called "sensor MV" or "sensor VE").
EP-A-0 804 939 describes a pacemaker in which the signals delivered by a minute ventilation sensor are used in addition to a control function, to diagnose the decompensation (i.e., the deterioration or worsening) of the cardiac insufficiency and to cause a modification of the programming of the pacemaker.
One problem with this device is that it presents the disadvantage of not taking into account the real level of activity of the patient. Indeed, it is known that persons with cardiac insufficiency have a deteriorated ventilatory function because of their pathology. As a result, these patients will present, during an effort level of activity (i.e., a level of activity above rest), an oxygen consumption (VO.sub.2) which is lower than that of healthy patients; on the other hand, the heart rate and minute ventilation of these patients will increase significantly more than is the case for healthy patients, for the same level of activity.
Thus, a relative or absolute variation in time of the signals detected by the minute ventilation sensor, or in a more general way, by a different physiological sensor which is provided to give an adequate representation of the metabolic needs of the patient, can be due to cardiac insufficiency or to a simple change of the activity level of the patient.
It also has been proposed, for example, in EP-A-0 750 920 and its corresponding U.S. Pat. No. 5,722,996 (commonly assigned to the assignee of this application, ELA Meedical, Montrouge, France) to combine the information delivered by two sensors, one physiological (typically a minute ventilation sensor), the other of physical activity (typically an accelerometer, often called "sensor G"). The two sensors operate a so-called "cross monitoring" of their respective indications, and the pacemaker combines the information from the two sensors as described therein to control the heart rate. However, this device also does not take into account the evolution over the long term of the pathology of the patient, who can present greater or lesser cardiac decompensation over time.
This pathology can indeed evolve according to various factors such as the effectiveness of cardiac stimulation, the effectiveness of the drugs delivered, or the food, or the presence of atrial and/or ventricular arrhythmias.